Technical Field
The present invention relates to a nanocomposite membrane with a Si—Y nanocomposite layer on an alumina support. The present invention also relates to a dip coating method of making the nanocomposite membrane and a method of separating a mixture of gases with the nanocomposite membrane.
Description of the Related Art
The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly or impliedly admitted as prior art against the present invention.
Many believe that the world will have a hydrogen based economy in the future. Hydrogen gas combines with oxygen gas to produce water, and in the process releases a substantial quantity of energy:H2(g)+½O2(g)(spark)→H2O(g) ΔH=−242 kJ/mol
Unlike electricity, hydrogen is an advantageous energy source as it can be transmitted by pipelines over long distances and it consumes less energy to transmit than the same amount of energy in the form of electricity. Hydrogen is widely used in various fields of industries, such as chemical, steel, and oil refining and petro-chemical industries. It is also a clean alternative energy source, leading many to predict an increase in hydrogen demand in the near future. Hydrogen is the simplest element and most plentiful gas in the universe. Yet hydrogen never occurs by itself in nature, and always combines with other elements such as oxygen and carbon. Hydrogen can be separated from the mixed gas using one of several adsorption technologies such as pressure swing adsorption (PSA). Due to the higher operation temperature and several complicated purification steps, the cost to produce H2 with these current technologies is too high for it to be an alternative to conventional hydrocarbon fuels. About 50% of world's hydrogen is produced from natural gas CH4 by means of steam-reforming reactions, which are commonly operated at around 800° C. If hydrogen (H2) can be selectively removed from the reforming reactor through a H2-permselective membrane, the thermodynamic equilibrium can be shifted to the product side, resulting in a higher conversion of CH4 to H2 even at lower temperatures (˜500° C.). Microporous ceramic membranes with molecular sieve-like properties for gas separation have attracted tremendous attention because of their high durability at elevated temperatures and in severe corrosive environments compared with polymer membranes, See A. J. Burggraaf, “Fundamentals of Inorganic Membrane Science and Technology”, Membrane Science and Technology Series, Vol. 4. Elsevier, Amsterdam, (1996), incorporated herein by reference in its entirety.
In particular, start-of the-art microporous silica membranes have been reported to separate hydrogen from other larger gas molecules by a simple molecular sieving effect. The multilayer membrane reactors can be used for methane-reforming reactions for hydrogen production and conversion enhancement in dehydrogenation chemistry, See T. Tsuru, K. Yamaguchi, T. Yoshioka, and M. Asaeda, “Methane Steam Reforming by Microporous Catalytic Membrane Reactors”, AIChE J., 50, 2794-805 (2005), incorporated herein by reference in its entirety. The pore diameter and the thickness of the membranes have been successfully controlled via methods including chemical vapor deposition (CVD), sol-gel techniques, and polymer precursor methods, See M. Nomura, H. Aida, S. Gopalakrishnan, T. I Sugawara, S. Nakao, S. Yamazaki, T. Inada and Y. Iwamoto, “Steam stability of a silica membrane prepared by counter diffusion chemical vapor deposition”, Desalination 193, 1-7 (2006); R. M. de Vos and H. Verweij, “High-Selectivity, High-Flux Silica Membranes for Gas Separation”, Science, 279, 1710-1711 (1998); Y. Iwamoto, K. Sato, T. Kato, T. Inada, Y. Kubo, “A hydrogen-permselective amorphous silica membrane derived from polysilazane”, J. Euro. Ceram. Soc. 25, 257-264 (2005), each incorporated herein by reference in their entirety. Ultra microporous silica membranes can be synthesized by sol-gel or CVD methods and are cheap to produce. However, there is often a trade-off between permeability and selectivity in both types of membranes, as sol-gel membranes deliver higher fluxes while CVD membranes higher selectivities.
Recently, oxide-based nanoparticle-dispersed amorphous silica membranes have been synthesized and designed also for gas separation applications in industries such as coal gasification, steam methane reforming, water-gas shift reaction and fuel cell systems, See M. Kanezashi and M. Asaeda, “Hydrogen permeation characteristics and stability of Ni-doped silica membranes in steam at high temperature”, J. Membr. Sci. 271, 86-93 (2006); L. Barelli, G. Bidini, F. Gallorini, S. Servilli, Hydrogen production through sorption-enhanced steam methane reforming and membrane technology: a review, Energy 33, 554 (2008), each incorporated herein by reference in their entirety. Typically these industries may require gas separation at temperatures preferably between 100° and 500° C., which can be met by the employment of silica or metal-based membranes. In the last two decades, gas-separation membranes have been developed using various materials, such as palladium and its alloys, silica and alumina, etc. Palladium membranes formed on porous alumina supports by the electro-less plating techniques and the MOCVD methods were reported to show high H2-permselectivity at 300°-500° C., but they have some disadvantages, such as degradation of H2-separation performance, for example, in the presence of hydrocarbons at high temperatures, See E. Kikuchi, Palladium/Ceramic Membranes for Selective Hydrogen Permeation and Their Application to Membrane Reactor, Catal. Today, 25, 333-337 (1995), incorporated herein by reference in its entirety. In order to improve the stability of Pd membranes against the chemical poisoning and mechanical load, Pd alloys (Pd—Ag, Pd—Ni, Pd—Nb) have been employed for metal membranes. In addition to problems with cost, these Pd alloy membranes still have high temperature stability problems, See C.-S. Jun, K.-H. Lee, Palladium and palladium alloy composite membranes prepared by metal-organic chemical vapor deposition method (cold-wall), J. Membr. Sci. 176, 121 (2000), incorporated herein by reference in its entirety.
One of the major concerns with silica-derived membranes relate to their structural integrity and stability for wet gas stream exposure which is generally the case in industrial gas processing. The morphology of silica has been shown to alter upon exposure to steam, mainly directly attributed to the collapse of small pores and expansion of larger pores, resulting in a loss of selectivity, See S. Giessler, L. Jordan, J. C. Diniz da Costa, G. Q. Lu, Performance of hydrophobic and hydrophilic silica membrane reactors for the water gas shift reaction, Sep. Purif. Technol. 32 (1-3), 255 (2003); I. Himai, H. Morimoto, A. Tominaga, H. Hiraschima, Structural changes in sol-gel derived SiO2 and TiO2 films by exposure to water vapour, J. Sol-Gel Sci. Technol., 10, 45. (1997); R. K. Iler, The Chemistry of Silica; Wiley & Sons: New York, (1979), each incorporated herein by reference in their entirety. To address this problem, some research groups have incorporated templates into the silica film whereby the membrane films were calcined in non-oxidizing atmospheres, See G. P. Fotou, Y. S. Lin and S. E. Pratsinis, Hydrothermal stability of pure and modified microporous silica membranes, J. Mater. Sci. 30, 2803-2808 (1995), incorporated herein by reference in its entirety. As a result, template silica membranes imparted hydro stable properties, accompanied by structural modifications. For instance, gas selectivity was lowered as covalent ligand methyl template in methyl triethoxysilane led to the formation of slightly larger pore sizes, See M. Nomura, K. Ono, S. Gopalakrishnan, T. Sugawara, and S.-I. Nakao, Preparation of a Stable Silica Membrane by a Counter Diffusion Chemical Vapor Deposition Method, J. Membr. Sci., 251, 151-8 (2005), incorporated herein by reference in its entirety. It has been reported that silica membranes prepared by using CVD technique from methyltriethoxyxilane showed high hydrothermal stability with a steam pressure of 75 kPa at 500° C., See M. Nomura, H. Aida, S. Gopalakrishnan, T. Sugawara, S-I. Nakao, S. Yamazaki, T. Inada, Y. Iwamoto, Steam stability of a silica membrane prepared by counter-diffusion chemical vapor deposition, Desalination 193, 1-7 (2006), incorporated herein by reference in its entirety. Moreover, it was found that γ-Al2O3-doped silica composite membranes can withstand hydrothermal conditions because of the strong network generated by the formation of Si—O—Al linkages, See Y. Gu, P. Harcarlioglu and S. T. Oyama, Hydrothermally stable silica-alumina composite membranes for hydrogen separation, J. Membr. Sci. 310, 28-37 (2008), incorporated herein by reference in its entirety. The use of hexyl tri-ethyl ammonium bromide has also improved the hydrothermal stability of membranes when the materials were prepared by sol-gel processes, See M. C. Duke, J. C. Diniz da Costa, G. Q. (Max) Lu, M. Petch, and P. Gray, Carbonized Template Molecular Sieve Silica Membranes in Fuel Processing Systems: Permeation, Hydrostability and Regeneration, J. Membr. Sci., 241, 325-33 (2004), incorporated herein by reference in its entirety. The presence of carbon in the silica matrix can create a hydrophobic surface, which can control the free motion of silanol groups in the silica network at high temperatures and eventually reducing the densification of the silica network, See R. Igi, T. Yoshioka, Y. H. Ikuhara, Y. Iwamoto, and T. Tsuru, Characterization of Co-Doped Silica for Improved Hydrothermal Stability and Application to Hydrogen Separation Membranes at High Temperatures, J. Am. Ceram. Soc., 91, 9, 2975-2981 (2008), incorporated herein by reference in its entirety In order to improve the stability of silica membranes in steam, inorganic oxides, such as TiO2, ZrO2, Fe2O3, Al2O3, NiO, etc. were also tested by doping with silica, See C. H. Chang, R. Gopalan and Y. S. Lin, A Comparative Study on thermal and Hydrothermal Stability of Alumina, Titania and Zirconia Membranes, J. Membrane Sci., 91, 27-45 (1994), incorporated herein by reference in its entirety. Of these metal-doped membranes, Ni-doped silica membranes showed relatively high H2-permence and high stability against water vapor at 35-300° C., suggesting the effectiveness of the addition of nickel oxides to silica for the membrane stability against steam at higher temperatures. Kanezashi and Asaeda found the incorporation of 33% nickel doping to improve hydrothermal stability, See M. Kanezashi and M. Asaeda, “Hydrogen permeation characteristics and stability of Ni-doped silica membranes in steam at high temperature”, J. Membr. Sci. 271, 86-93 (2006), incorporated herein by reference in its entirety. However, both hydrogen and nitrogen permeance decreased by 60% and 93%, respectively upon exposure to high temperature water vapor, favoring high H2/N2 selectivity. Duke et al. reported improved gas selectivity by exposing to steam silica membranes with carbonized templates derived from C6 surfactant triethylhexyl ammonium bromide, See M. C. Duke, J. C. Diniz da Costa, D. D. Do, P. G. Gray, G. Q. Lu, Hydrothermally robust molecular sieve silica for wet gas separation, Adv. Func. Mater. 16, 1215-20 (2006), incorporated herein by reference in its entirety. Gu et al. achieved enhanced hydrothermal stability using 3% alumina doping in CVD membranes, however, 45% hydrogen permeance reduction was observed after long-term exposure to steam, See Y. Gu, P. Harcarlioglu and S. T. Oyama, Hydrothermally stable silica-alumina composite membranes for hydrogen separation, J. Membr. Sci. 310, 28-37 (2008), incorporated herein by reference in its entirety.
Nanocrystalline Y2O3 has been mostly investigated for applications in the field of de-NOx catalysts and phosphors for lighting, See M. Fokema and J. Y Ying, The selective catalytic reduction of nitric oxide with methane over scandium oxide, yttrium oxide and lanthanum oxide, Appl. Catal. B. 18, 71-77 (1998); Md. H. Zahir, T. Suzuki, Y. Fujishiro, and M. Awano, Synthesis and characterization of Sm3+-doped Y(OH)3 and Y2O3 nanowires and their NO reduction activity, J. of Alloys and Comp. 476, 335-340 (2009); C. Cannas, M. Casu, A. Musinu, G. Piccaluga, A. Speghini and M. Bettinelli, Synthesis, characterization and optical spectroscopy of a Y2O3—SiO2 nanocomposite doped with Eu3+, J. Non-Crystalline Solids 306, 193-199 (2002), each incorporated herein by reference in their entirety. The development of Y2O3 thin films has aroused great interest for their excellent properties such as high dielectric constant, low absorption, chemical stability and excellent thermal conductivity make them adequate for numerous technological applications. Different properties can be observed in rare earth doped films compared with bulk materials, making them interesting to study. Zahir et al. reported that Y2O3-alone showed quite high and stable catalytic activity for NO reduction at 500° C. in the presence of water vapor, See Md. H. Zahir, T. Suzuki, Y. Fujishiro, and M. Awano, Synthesis and characterization of Sm3+-doped Y(OH)3 and Y2O3 nanowires and their NO reduction activity, J. of Alloys and Comp. 476, 335-340 (2009), which is incorporated herein by reference in its entirety. Boffa et al. recently developed a niobia-silica mixed oxide microporous membrane that combines the open percolative pore structure of silica with increased thermal stability, and has a high selectivity for CO2, See V. Boffa, H. L. Castricum, R. Garcia, R. Schmuhl, A. V. Petukhov, D. H. A. Blank, and J. E. ten Elshof, Structure and Growth of Polymeric Niobia-Silica Mixed-Oxide Sols for Microporous Molecular Sieving Membranes: A SAXS Study, Chem. Mater., 21 (9), 1822-1828 (2009), incorporated herein by reference in its entirety.
In view of the forgoing, the objective of the present invention is to provide a nanocomposite membrane with a Si—Y nanocomposite layer on an alumina support, a dip coating method of making the nanocomposite membrane, and a method of separating a mixture of gases with the nanocomposite membrane.